1. Field of the Invention
The present invention relates to an optical coupler, especially to a prism-nonlinear optical crystal coupler for laser frequency conversion.
2. Description of Related Art
At present, Laser frequency conversion with nonlinear optical crystal, which includes frequency doubling, sum- and difference-frequency, parameter oscillation and amplification, is achieved as following. At first, the nonlinear optical crystal is cut according to the phase-matching angle. The fundamental laser beam is then incident onto the nonlinear optical crystal at a certain incident angle. Subsequently, the incident angle is adjusted to the phase-matching angle (called angle tuning), or the crystal temperature is controlled the phase-matching temperature, in order to achieve efficient conversion, as described by I. V. G. Dmitriev, G. G. Gurzadyan, D. N. Nikogosyau in “Handbook of nonlinear optical crystals”, second revised and updated edition, Spring-verlag, Berlin, Heidelberg, New York, London, Paris, Tokyo, Hong Kong, 1996. The drawback of this method is that some of the nonlinear optical crystals are not easy to be cut, some of them do not have enough size and some of them are too expensive to be cut into certain shapes to meet the requirement of the phase match.